1645383721 has 4 divisors (see below), whose sum is σ = 1650640852. Its totient is φ = 1640126592.

The previous prime is 1645383671. The next prime is 1645383739. The reversal of 1645383721 is 1273835461.

It is a semiprime because it is the product of two primes.

It can be written as a sum of positive squares in 2 ways, for example, as 596336400 + 1049047321 = 24420^2 + 32389^2 .

It is a cyclic number.

It is not a de Polignac number, because 1645383721 - 2¹⁵ = 1645350953 is a prime.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (1645303721) by changing a digit.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 2628096 + ... + 2628721.

It is an arithmetic number, because the mean of its divisors is an integer number (412660213).

Almost surely, 2^1645383721 is an apocalyptic number.

It is an amenable number.

1645383721 is a deficient number, since it is larger than the sum of its proper divisors (5257131).

1645383721 is an equidigital number, since it uses as much as digits as its factorization.

1645383721 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 5257130.

The product of its digits is 120960, while the sum is 40.

The square root of 1645383721 is about 40563.3297573067. The cubic root of 1645383721 is about 1180.5627214071.

The spelling of 1645383721 in words is "one billion, six hundred forty-five million, three hundred eighty-three thousand, seven hundred twenty-one".